A stent refers to a mesh tube implanted when the inner diameter of the artery or the blood vascular system becomes so narrow due to deposition of thrombus or lipids in the coronary arteries and the peripheral blood vessels that the flow of the bloodstream is not smooth, or when tumors occur in the non-vascular system such as the gastrointestinal tract, the esophagus, and the respiratory tract, or against stenosis occurring after a surgery. Conventionally, stents were manufactured by using metal or silicone materials, but these stents remain permanently in the human body, and thus need to be subjected to removal surgery because the remaining stents cause inflammation or other diseases.
In order to solve these problems, natural polymers or synthetic polymers and the like having biodegradability have been used. Biodegradable natural polymer materials have not only limited physical properties, but also problems in processability and mass-producibility, and thus have limitations in use. Meanwhile, synthetic polymers are generally used due to non-toxicity to human body, excellent mechanical properties and adjustable biodegradability. Accordingly, there have been extensive researches on biodegradable synthetic polymer materials. In particular, aliphatic polyesters having excellent physico-mechanical and hydrolytic characteristics have been the focus of various researches.
However, since biodegradable polymers have a much lower mechanical strength than metals and ceramics, the use thereof is limited. The low strength of biodegradable polymers is also due to a nature of polymer material, but is incurred by preparation methods thereof. That is, when a melt processing method such as extrusion, injection and compression moldings is used during the molding of a polymer, the breakdown of molecular chains is formed in a significant amount, and thus the molecular weight of the polymer is decreased, thereby reducing the final mechanical strength (Pierre Erwan Le Marec et al., Polym. Deg. Stab., 110, 353 (2014)).
Accordingly, various methods have been proposed in order to increase the mechanical strength of a biodegradable polymer.
For example, Korean Patent Publication No. 2001-0100249 A discloses a preparation method comprising a two-step process of vacuum compression molding and solid extrusion to reduce a decrease in molecular weight resulting from thermal degradation of biodegradable polymers. This method also describes process parameters adjustment to satisfy strength requirements of the polymer. The process parameters include a crystallinity of a vacuum compression molded product, a drawing ratio or a drawing speed and the like.
Meanwhile, Jeffrey S. Wiggins et al. suggested a method for modifying degradation of biodegradable polyesters, in which hydroxyl end group of the polymer is substituted with carboxylic acid. In this study, Wiggins et al showed that carboxylic acid end group functionality has an effect on degradation rate of the polymer and may reduce the weight loss compared to their hydroxyl-terminated analog (Jeffrey S. Wiggins et al., Polymer, 47, 1960 (2006)). However, since the hydroxyl end group is substituted by adding an acid anhydride as one of the polymerization components, the Wiggins' method has a problem in that the polymerization process becomes complicated.
Further, Lee et al. described the effect of different end group functionality of biodegradable polymer on degradation properties such as molecular weight reduction (Soo-Hong Lee et al., Journal of Polym. Sci. Part A: 39, 7 (2001)). For this purpose, Lee et al. prepared biodegradable polymers with various functional groups via pre-treatment process using substitution catalyst and analyzed thermal stability and thermal degradation rate etc. However, since a functional group is substituted by using a catalyst during the solution polymerization, this method has a problem in that the polymerization process becomes complicated.